MXPA03006452A - Method for well logging using nuclear magnetic resonance and device for carrying out said method. - Google Patents

Abstract

The invention relates to methods for prospecting wells using nuclear magnetic resonance. The inventive method consists in generating a static magnetic field near a well with the aid of a magnet which is made of a conductive rear-earth material in the form of an elongated parallelepiped which is magnetised in a direction perpendicular to a large side surface and longitudinal axis thereof. The width of the magnet is of at least twice as large as the narrow side of said parallelepiped. A radio-frequency coil is arranged on a frame whose diameter is equal to or higher than a diagonal of the cross-section of a magnet arranged inside the frame. The coil elements are arranged on planes which are parallel to the longitudinal axis of the magnet and vertical with respect to the smaller side thereof arranged in symmetrical sectors which are opposite to the large side surface of the magnet. A compensating unit is arranged along the large side surfaces of the magnet parallel to the longitudinal axis thereof.

Description

METHOD FOR POLLING DIAGRAFY USING NUCLEAR MAGNETIC RESONANCE AND DEVICE FOR EFFECT SUCH METHOD FIELD OF THE INVENTION The invention relates to geophysical methods for detecting wells, and in particular to nuclear magnetic sounding (NML) diagraphy used for the study of oil and natural gas wells.

BACKGROUND OF THE INVENTION There are NML methods that use intense solenoid magnets to generate a static magnetic field in an area located near the perforation wall, and which generate a radiofrequency field in this area, predominantly perpendicular to the static magnetic field. , and to receive nuclear magnetic resonance signals [1, 2]. However, these methods have not gained a wide application. There is a NML method that includes the use of a magnet focusing system to generate a uniform static magnetic field in the area opposite the magnet system in direct proximity to the perforation wall, to generate a radio frequency field in this area, the direction of the radiofrequency field being predominantly perpendicular to the static magnetic field, and to receive this nuclear magnetic resonance signal coming from this area [3]. The shortcoming of this method is the depth of the detection zone, which is located at a distance in the order of 3 cm from the wall of the probe. In perforations with empty spaces, the detection region is in the area of the perforation, which leads to false results. The closest to the technical solution for which the request is made is the nuclear magnetic sounding survey method which includes the generation of a static magnetic field close to the borehole in the region to be analyzed, using at least one field magnetic field with a long longitudinal axis and a magnetization direction running predominantly perpendicular to said axis, the generation of a radiofrequency field to excite the atomic nuclei of the material analyzed in said region, using at least one coil winding in such a way that the turns of the coil are on the axes predominantly parallel to said direction of magnetization and said longitudinal ee, and the reception of the nuclear magnetic resonance signals coming from the excited cores to obtain information about the properties of the material analyzed [4, sections 1, 6]. The device that uses this method consists of a circular cylindrical magnet made of ferrite and magnetized perpendicular to its long axis, and a radiofrequency coil wound directly on the magnet, resting the windings of the coil mainly in the plane passing through the axis of the magnet and the direction of its magnetization [4, sections 14, 15, 16], a radio pulse generator, a nuclear magnetic resonance signal receiver and an equalizer device; the start of the radiofrequency coil is connected to the first input of said equalizing device, and the end of said coil is connected to the common point in the equalizing device, while the output of the equalizing device is connected to the input of the signal receiver magnetic resonance imaging [4, 4]. The advantage of this method and device is that the detection zone is in a thin cylindrical region (of the order of 1 mm) coaxial with the axis of the probe, at a significant distance from its axis. For a probe with a diameter of 152 mm, the detection zone is located at a distance of 175 mm from its axis, and hardly in the region of the perforation with the standard diameter of 200 mm [5]. defect is that, as the diameter of the probe is reduced (for example up to 114 mm), the sensitivity of the probe and the radius of the investigation area are reduced, and therefore, it can only work in diameter boreholes small [5]. This defect is caused by the fact that the device prototype [4] uses a circular cylindrical non-conductive ferrite magnet, while the radiofrequency coil is wound directly on the surface of the magnet [4, sections 14, 15, 16] .

BRIEF DESCRIPTION OF THE INVENTION The proposed invention solves the problem of increasing the sensitivity and depth of detection of nuclear magnetic sounding using small diameter probes. The indicated problem is solved in the following way. In the nuclear magnetic sounding diagnostic method that includes the generation of a static magnetic field in the borehole, in the region that will be analyzed, using one of several magnets made of a rare earth conductive material with long longitudinal axis and a direction of magnetization that runs mainly perpendicular to said axis, the generation of the exciter radiofrequency field in the direction perpendicular to both of said axes and the static magnetic field, and the reception of nuclear magnetic resonance signals from the excited nuclei, and also generates a compensating radiofrequency field in the area of the magnet. In addition, in the nuclear magnetic sounding logging apparatus consisting of at least one long magnet, magnetized perpendicular to its long axis, and a radiofrequency coil that creates a field perpendicular to the field of the magnet, a radio pulse generator, a nuclear magnetic resonance signal receiver and an equalizing device, with the start of the radiofrequency coil connected to the first input of the equalizing device and the end of said coil connected to the common point of the equalizing device, to the second input of which is connects the output of the radio pulse generator, while the output of the equalizing device is connected to the input of the nuclear magnetic resonance signal receiver, the magnet is made of a rare earth conductive material in the form of a long parallelepiped. This is magnetized perpendicular to its long axis and the wide lateral surface. The width of the magnet is twice the width of its narrow side, while the radiofrequency coil is wound on a cylinder with a diameter at least equal to the diagonal of the cross section of the magnet located inside the cylinder. The turns of the coil rest on the planes parallel to the long axis of the magnet and perpendicular to its narrow side in symmetrical sectors positioned opposite the wide lateral surface of the magnet, while a compensating device is placed along the wide lateral surfaces of the magnet. magnet, parallel to its long axis. In addition, the compensating device comprises a coil with a start connected to the common point in the equalizing device, while its end part is connected to the end of the radio frequency coil. The ratio of the coils of the radiofrequency coil and of the compensating coil is equal to the ratio of the diameter of the radiofrequency coil to the thickness of the magnet. In addition, the compensating device comprises a short circuit coil made of a material with a resistivity of less than 2.5 x 10"6 ohm / cm. The innovation in this nuclear magnetic sounding logging method compared to the prototype is that compensation is proposed of the radio frequency field in the area of the magnet, for this, a radiofrequency field is also generated in the area of the magnet, directed towards an equal in intensity to the exciter radio frequency field in the area of the magnet. For nuclear magnetic sounding logging is that the magnet is made of conductive rare earth material in the form of a magnetized parallelepiped perpendicular to its long axis and wide lateral surface, while the radiofrequency coil is wound on a cylinder with a diameter not less than the diagonal of the cross section of the magnet located inside the cylinder The turns of the coil are in the The planes are parallel to the longitudinal axis of the magnet and parallel to its narrow side in symmetrical sectors located opposite the wide lateral surface of the magnet. A compensating device is placed along the wide side surfaces of the magnet, parallel to its long axis. Another innovation is that the compensating device comprises a coil with a start connected to the common point in the equalizing device, while its end is connected to the end of the radiofrequency coil. The ratio of the turns of the radiofrequency coil to the compensation coil is equal to the ratio of the diameter of the radiofrequency coil to the thickness of the magnet. Another innovation is that the compensating device comprises a short-circuit coil made of a material with a resistivity of less than 2.5 x 10"6 ohm / cm.The examination of known solutions in science and technology related to survey sounding methods using nuclear magnetic resonance and devices to implement it have shown that there is no identical solution.In the known devices and the closest prototype [4], there is no additional field that compensates with respect to the radio frequency field in the area of the magnet. repeated form in the sections of the claim that is applied to the device (see sections 14, 15, 16) and the methods for application of this (sections 19, 26) that the magnet is made in the form of a long circular cylinder of ferrite that have "non-conductive" properties, while the radio-frequency coil is wound directly on the magnet, the device can not work if it is not these requirements. Any of the materials (except dielectric materials) placed inside the radiofrequency coil lead to losses in the radio frequency coil. This manifests as a reduction in the Q factor of the coil at the resonance frequency and finally as a reduction in the signal to noise ratio of the output of the apparatus. Therefore, a ferrite-based magnet with "non-conductive" properties is used in the prototype device. Because the ferrite magnet has a low residual magnetization, it is made with a round shape to obtain the maximum field strength. The long circular cylindrical magnet creates a static magnetic field with intensity and direction at each point in the surrounding space, which can be determined using the following equation [6, 7]: H. - SEN. { 5 < p) +. (1) ? f = -H. { C0S. { 5 (p) + - in which Hr, H (p) are the radial and tangential components of the magnet field at a point with coordinates r, cp in a cylindrical coordinate system with an axis that coincides with the axis of the magnet, and H0 is the field strength on the surface of a magnet with radius At a distance r> 2E from the axis, the field of the magnet practically contains only the first harmonic of the expression (1) .This field is homogeneous (in magnitude) in the azimuthal direction in the fixed radius rp The quantity H0 is directly proportional to the magnitude of the residual magnetism of the magnet material Br. The ferrite has a residual magnetism Br = 3000-4000 gauss, and the rare earth material NdFeB has Br = 10,000-11,000 gauss In general, a detection zone rp = 170-180 mm is selected from the probe axis with field strength in this area of the order of 165-170 gauss for nuclear magnetic sounding [5]. a ferrite magnet with diameter 120 nm to obtain this field strength in this detection zone. However, a magnet made from NdFeB will have a diameter of 65-70 nm. Therefore, the change made to a NdFeB magnet creates the prerequisites for manufacturing a smaller magnetic diameter nuclear magnetic sounding probe without reducing the radius of the detection zone. A magnet made from ferrite is "non-conductive". The radiofrequency coil wound on its surface has a Q factor of the order of 100 [5]. A magnet made from NdFeB conducts the current better, and the radio frequency coil winding on its surface will have a Q factor not greater than 20. To reduce the losses in the radio frequency coil, it is necessary to reduce the cross-sectional area of the material inserted inside the radiofrequency coil and intersect the electromagnetic flux of said coil, and eliminate the radio frequency field in the area of the material inserted inside the coil. The proposed novel method and apparatus for nuclear magnetic sounding makes it possible to solve these problems. It is proposed to compensate the radiofrequency field in the area of the magnet to eliminate the additional losses caused by the change in the material of the magnet. However, the field compensation in the area of the magnet leads to a decrease in the exciter radio frequency field in the area of the analyzed substance. If the radii of the radio frequency coil and the compensation coil are equal, there will be no field in the area of the magnet, but there will be no field in the detection zone either. It is necessary to reduce the cross section of the magnet intersected by the radiofrequency field in comparison with the radius of the radio frequency coil. For this, it is proposed to change the shape of the magnet in the proposed invention. The magnet becomes thinner in the plane perpendicular to the direction of flow of the radiofrequency coil and wider in the direction of magnetization of the magnet. The field of a magnet in the form of a long parallelepiped is described by an expression analogous to (1). However, the shape coefficients, which are functions of the ratio of the wide and narrow walls of the magnet, appear in front of each of the terms of the series (1). For the first harmonic of the field, which is of interest, at a distance r = rp this relation has the form: H = n | SEN ((p) D (2) h? f = -? 0 R | KOS. { ) D r in which h is the narrow lateral surface of the magnet, D is the wide lateral surface of the magnet, a H0 is the field strength of the magnet on its narrow lateral surface.

For a wide lateral surface 2 times wider than the narrow lateral surface, the field strength in the detection probe will be 2 times greater than that of a circular cylindrical magnet with a diameter equal to the narrow lateral surface. You can get a similar field strength with a circular cylindrical magnet with diameter -J2 times greater than the width of the narrow side surface of a rectangular magnet made from the same material. Therefore, if a magnet is made from NdFeB in the form of a parallelepiped with a narrow side of 40 mm and a wide side of 80 raí, with longitudinal axis length of 1000 mm, and magnetized perpendicular to the axis throughout of the length and side side, then said magnet will have a field in a detection zone rp = 170-180 mm analogous to that of a circular cylindrical ferrite magnet of 120 mm diameter of the same length. The radiofrequency coil for a probe with a magnet in the form of a parallelepiped is wound on a cylinder with a diameter not less than the cross section of the rectangular magnet. The turns of the coil are placed along the generatrix of the cylinder along its long axis in symmetrical sectors with an angle 2d placed in opposition to the wide side of the reclanar magnet. The intensity and direction of the field of the radiofrequency coil can be determined from an equation analogous to (1): · SEN (3 { < p + 90 °)) + (3) SEN. { 5) - \ ^ | COS (f + 90 ° + ^ SEN (3d) - ^ \ - COS (? (< p + 90 °)) +? \ F = -? + -SEN (5S) - \ - \ | COS (5 (< p + 90 °)) + · · | 5 r in which Hlr, Hl (p) are the radial and tangential components of the radiofrequency field strength at a point with coordinates r, cp in a cylindrical coordinate system with an axis that coincides with the axis of the magnet. The radiofrequency coil at all points in the surrounding space is rotated 90 ° relative to the field of the magnet.Hl0 is the radiofrequency field strength in the wall of the radiofrequency coil. iw p dR (4) is the current density through a p · d radio frequency coil with a radius R and a number of turns W. A radio frequency coil operates more efficiently when the turns are accommodated as length of a cylinder in a sector of 120 °. When 2d = 120 °, the second harmonic in equation (3) is equal to zero. Therefore, the radiofrequency field in the azimuth direction is practically uniform within the radius of the detection zone rp. In addition, the energy coming from the radio pulse generator is mainly consumed to create the second, useful harmonic of the radio frequency field. The losses in the radiofrequency coil will be identical when placing a flat magnet and a cylindrical magnet with a diameter equal to the narrow side of the flat magnet inside the radiofrequency coil. The field strength of the flat magnet will be 2 times greater. The difference between the diameter of the radiofrequency coil and the thickness of the flat magnet makes it possible to use a compensating coil that is wound directly on the magnet in the plane parallel to its narrow side. The compensating coil is connected opposite to the primary radio frequency coil. Here, for the complete compensation of the radiofrequency field in the area of the magnet, as follows from expression (4), the ratio of the turns of the compensating coil and the radiofrequency coil must be equal to the ratio of their radii . The absence of a radio frequency field in the area of the magnet leads to the absence of losses in the radiofrequency coil caused by the presence of the magnet inside it. The intensity of the useful radiofrequency field in the detection zone is insignificantly reduced here, as shown in Figure 4. For example, if the thickness of the magnet is 40 mm, while the diameter of the radiofrequency coil is 100 mm, then the radiofrequency field in the operating area of the probe will be reduced by only 16%. The apparatus proposed to compensate for the losses in the radio frequency coil works efficiently, but in some cases it is difficult to manufacture. For the high frequencies at which the NML is used, the radio frequency coil has few turns. Thus, it is difficult to select the turns of the compensating coil. In this case, a round in short circuit is mounted on the magnet. A cover made of highly conductive material applied to the surface of the magnet (a thin sheet copper cover, for example) serves as the short circuit return. This statement is confirmed by the experimental data provided in Figure 5. Physically, the cover functions as a compensating coil. Currents coming from the radio frequency coil are induced in the cover, and these compensate with respect to the radiofrequency field in the area of the magnet. Therefore, the combination of the change in the shape and material of the magnet, the design of the radiofrequency coil and the addition of a compensating device makes it possible to obtain a new quality, specifically, the detection zone and the sensitivity of the instrument remain No change, but the diameter of the probe is smaller.

All the above shows that the invention for which an application is made is novel, it is an innovation and can be used to create nuclear magnetic sounding probes.

BRIEF DESCRIPTION OF THE FIGURES The technological essence of the invention is explained using the figures, in which: Figure 1 shows a block diagram of the NML apparatus, - Figure 2 shows the general view of the NML probe, - Figure 3 shows the cross section of the NML probe; Figure 4 shows the variations of the field of the radiofrequency coil, the field of the compensating coil and the sum of the radio frequency field as functions of the distance from the narrow lateral surface of the magnet; Figure 5 shows the experimental data obtained at a frequency of 500 kHz using a radiofrequency coil of 100 mm diameter with four turns, inside a magnet made from NdFeB of variable thickness and the same magnet is inserted with a 2 O cover made of different materials.

DESCRIPTION OF THE PREFERRED MODALITY The apparatus for nuclear magnetic sounding logging is designed as follows: this comprises the probe for nuclear magnetic sounding 1, the equalizing device 2, the radio pulse generator 3 and the receiver 4. Nuclear magnetic sounding logging probe 1 comprises a long magnet constructed in the shape of a parallelepiped 5 and magnetized perpendicular to its long axis and wide side. The magnet is inserted into the cylindrical frame 6, on which the radiofrequency coil 7 is wound. The coil is wound in symmetrical sectors of 120 ° placed in opposition to the wide side of the magnet 5. The turns of the radiofrequency coil 7 rest in the planes parallel to the narrow side of the magnet 5. The compensating coil 8 with turns parallel to the turns of the radiofrequency coil 7 is wound on the magnet 5. The start of the radio frequency coil 7 is connected to the first input of the device equalizer 2, while its end is connected to the end of the compensating coil 8. The start of the compensating coil is connected to the common point of the equalizing device 2. the output of the radio pulse generator 3 is connected to the second input of the device equalizer, while the output of the equalizer device is connected to the input of receiver 4. The magnet is made from NdFeB in the form of a parallelepiped of 1000 mm long, 80 mm wide, with a narrow lateral side of 40 mm. The magnet is magnetized perpendicular to the long axis and to the wide lateral surface. A copper sheet cover 0.5 mm thick is applied to the surface of the magnet and covers its side surface along its entire length. The radiofrequency coil is made of a cylinder of laminated material with a fiberglass base 80 mm long with an internal diameter of 100 mm and an outer diameter of 102 mm. The turns of the coil are applied to the outer surface of the cylinder along its length, in symmetrical sectors of 120 °. The ends of the windings are also on the outside of the cylinder. The radio frequency coil is connected to the input of the equalizer device. A magnet is inserted into the radiofrequency coil so that the coil turns are opposite its wide lateral surface. The apparatus for nuclear magnetic sounding diagraphy works in the following way. The magnet 5 induces a static magnetic field parallel to the plane with magnitude HO at a distance rp from the e e of the magnet. The field magnitude HO is constant across the complete circle with radius rp. The direction of this field differs at different points in the circle. The radiofrequency coil 7 together with the compensating coil 8 generate a radiofrequency field sum parallel to the plane El, which has the same constant magnitude at the radius rp. The direction of the radiofrequency field Hl is perpendicular to the HO field at each point of a circle of radius rp. When the frequency of the radiofrequency field Hl equals the precession frequency of the hydrogen nucleus in the HO field in the detection zone 9, the phenomenon of nuclear magnetic resonance is presented. The nuclear magnetic resonance signal is collected by the same radiofrequency coil 7. The radio frequency field sum 10 consists of the field 11 created by the radio frequency coil and the field 12 created by the compensating coil. There is no radio frequency field in the area of the magnet, and the field varies insignificantly in the detection zone 9. A cover made of a highly conductive material applied to the surface of the magnet can serve as the compensating coil. In this case, the radiofrequency field sum varies according to the same law as in 10. The change in the Q factor of the radio frequency as a function of the material and width of the cover is shown in: 13 - a magnet made to from NdFeB without a cover; 14 - an elaborated steel roof with a resistivity of 42 x 10"s ohm / cm; 15 - an aluminum cover with resistivity of 2.5 x 10" s ohm / cm; 16 - an elaborated copper cover with resistivity of 1.55 x 10_s ohm / cm. As can be seen from the figure, the changes in the Q factor are smaller when a material with resistivity of less than 2.5 x 10"6 ohm / cm is placed inside the coil, and larger when placed inside the coil. coil a magnet made from NdFeB without a cover.

If the NdFeB-based magnet is covered with thin copper foil, the radiofrequency coil will only detect the thin copper foil.

Commercial Application The absence of a radio frequency field in the area of the magnet makes it possible to use any materials for the magnet, including rare earth conducting materials, for example NdFeB. Because the rare earth magnets have a significantly higher residual magnetization than the ferrite, the NML probe may have a smaller diameter but retain the same detection radius. An NML probe is evaluated in perforations up to 4500 meters deep at temperatures up to 120 ° C. It is possible to use the invention in high temperature perforations.

References cited 1. - Patent E.U.A. No. 3,667,035, class EUA 324/05, 1972. 2. - Patent E.U.A. No. 4,35,955, class EUA 324/303, 1982. 3.- Patent E.U.A! No. 5,055,787, EUA class 324/303, 1991. 4. - Patent E.U.A. No. 4,710,713, class EUA 324/05, 1987 [prototype]. 5. - .IST. Chandler, E.O. Drak, M.N. Miller and M.G. Prammer Improved Log Quality ITU to Dual Frequency Pulsed M R Tool. SPE 28365 Presented at the 69th Annual Technical Conference and Exhibition of SPE, 1994. 6. - V.A. Govorkov. Ehlektricheskie i magnitnye polya (Electrical and Magnetic Fields).

Moscow, Energy Press, 1968, 488 p. 7. - R.V. Grechiskin, L.E. Afanasieva, Yu G. Pastushenko and N.N. Maksimov. Analysis of a Linear Position Sensor with a Hall Effect Element.- Meas. Sci. Technol. , 1994, p. 853-860.

Claims (4)

1. NOVELTY OF THE INVENTION Having described the present invention is considered as a novelty and therefore the content of the following is claimed as property.
2. CLAIMS 1. - The method for sounding graphs using nuclear magnetic resonance that includes the generation of a static magnetic field near the hole using at least one long magnet with a direction of magnetization that passes predominantly perpendicular to the longitudinal axis of the magnet , the generation of a radiofrequency field in said region in the direction perpendicular to the longitudinal ee of the magnet and the static magnetic field, and the reception of the nuclear magnetic resonance signals, characterized in that the static magnetic field is generated using a magnet made to from a rare earth conductive material and also generates a field that compensates with respect to the radio frequency field in the area of the magnet. 2. - The apparatus for sounding logging using nuclear magnetic resonance comprising at least one long magnet magnetized perpendicular to its longitudinal axis and at least one radiofrequency coil with the coil turns resting in planes parallel to the longitudinal axis of the magnet and its direction of magnetization, an equalizing device, a radio pulse generator and a nuclear magnetic resonance signal receiver, with the radio frequency coil connected to the first input of the equalizing device, the output of the radio pulse generator connected to the second input of the equalizing device, and the output of the equalizing device connected to the input of the nuclear magnetic resonance signal receiver, characterized in that the magnet is made from rare earth conducting materials in the form of an elongated parallelepiped mag et. Iced perpendicular to its wide lateral surface. The width of the magnet is at least twice the width of its narrow side, while the radiofrequency coil is wound on a cylindrical frame. The diameter of the frame is at least equal to the diagonal of the cross section of the magnet located inside the cylindrical frame, the turns of the coil are accommodated in symmetrical sectors located opposite to the wide lateral surface of the material, while the magnet is equipped with an apparatus that compensates with respect to the radiofrequency field in the area of the magnet.
3. 3. The apparatus according to claim 2, characterized in that the device that compensates with respect to the radiofrequency field in the area of the magnet is a coil wound on the magnet, with turns resting in the planes parallel to the narrow surface of the magnet , the start of the compensating coil is connected to the common point in the equalizing device, and the end is connected to the end of the radio frequency coil, the ratio of the number of turns of the radiofrequency and compensator coils is equal to the ratio of the diameter of the radiofrequency coil to the thickness of the magnet.
4. 4. The apparatus according to claim 2, characterized in that the compensating device is a short circuit made of highly conductive material.